Particle radiation therapy (also called hadron therapy) is a form of external beam radiotherapy that uses beam(s) of energetic protons, ions, electrons, or electrons for treatment of cancer or other diseases. Further, pencil beam scanning (PBS) therapy is a type of particle radiation therapy that often allows excellent three-dimensional (3D) shaping of a high-dose radiation volume to a shape of a treatment target such as a tumor. With particle PBS therapy, the radiation treatment is delivered in several layers (beam energies or energy layers), where a pencil beam of energized particles is scanned to cover a cross section of the treatment target at specific energy-dependent depths.
However, during particle PBS therapy delivery, treatment target motion (e.g., tumor motion), for example, due to breathing or the heartbeat, will change the relative positions of treatment target volumes or spots that are irradiated sequentially by the scanning pencil beam. The size of the treatment target volumes or spots depends on a width of the pencil beam of energized particles. The resulting redistribution of dose in the treatment target tissue is called the interplay effect, and it can lead to severe local underdosage and/or overdosage inside the treatment target volume. This is a major concern for particle PBS therapy in the thorax and abdomen, in particular for stereotactic body radiotherapy (SBRT), where high doses are delivered in few fractions. Besides the interplay effect, treatment target motion (e.g., tumor motion) also shifts and smears the delivered target dose, but this smearing effect is common for particle PBS therapy and other treatment techniques, such as conventional photon radiotherapy and passively scattered particle beams. Unlike other motion-induced dose perturbations that impact the dose at the treatment target edge (e.g., dose blurring due to random position errors and dose shifts due to systematic position errors), the interplay effect typically cannot be accounted for by a simple expansion of the high-dose radiation volume with safety margins.
Although the interplay effect may tend to average out after several treatment deliveries because the hot spot and cold spot locations typically differ randomly between each treatment delivery, the interplay effect is more serious for treatments delivered in few fractions because the dose smearing effect of many fractions is absent. Since SBRT is delivered in few fractions, SBRT is susceptible to pronounced interplay effect in moving target organs, such as lung, liver, or pancreas. Consequently, many particle radiation therapy centers do not offer SBRT treatments at all. Even for normal-fractionated treatments, many particle radiation therapy centers only treat highly selected tumors with minor motion due to respiratory motion.